108 Chem
Unsaturated Hydrocarbons:
Alkenes
Chapter 3
1
Alkenes or Olefines
CnH2n
Crabon-carbon double bond
Nomenclature of Alkenes
Common names:
ane
IUPAC rules:
ylene
CH2=CH- vinyl group
CH2=CHCH2- allyl group
• Determine the parent name by selecting the longest chain that
contains the double bond and change the ending of the name of
the alkane of identical length from -ane to -ene.
• Number the chain so as to include both carbon atoms of the double
bond, and begin numbering at the end of the chain nearer the double
bond. Designate the location of the double bond by using the number
of the first atom of the double bond as a prefix.
2
• In cycloalkenes, the double is always found between carbon 1 and
carbon 2. It is therefore not necessary to specify the position of the
double bond with a number. If substituents are present, the ring must
numbered, starting from the double bond, in the direction that gives
the substituents the lowest number(s).
Examples
3
4
Hybridization in Alkanes:
5
Hybridization in Alkenes:
1.34A°
6
Geometric Isomerism in Alkene
Cis-Trans Isomerism
cis (identical or analogous
substituents on same side)
trans (identical or analogous
substituents on opposite sides)
7
If either vinyl carbon is bonded to two equivalent
groups, then no geometric isomerism exists.
CH3CH=CHCH3
yes
(CH3)2C=CHCH3
no
CH3CH2CH=CH2
no
CH3
CH3CH=CCH2CH3
yes
8
Examples:
H3C
CH3
C
C
cis-2-Butene
Cl
H
H
C
H
H
Cl
C
H3C
H
CH3
trans-2-Butene
Cl
C
H
C
H
cis-1,2-dichloroethene
C
H
C
Cl
trans-1,2-dichloroethene
9
The E-Z System
E:
Z:
Entgegen
higher ranked substituents on opposite sides
Zusammen
higher ranked substituents on same side
10
Examples:
H3 C
*
*
CH2CH3
C C
H
CH3
*
H3 C
Cl
C C
H
Z-3-methyl-2-pentene
E-1-bromo-1-chloropropene
Br
*
11
Physical properties of alkenes
A Physical States and Solubilities
C1-C4
gases
C5-C18
liquids
More than C18
solids
Alkenes are nonpolar compounds. Thus alkenes are soluble in
the nonpolar solvents such as carbon tetrachloride (CCl4) and
benzene (C6H6), but they are insoluble in polar solvents such as water.
12
Preparation of alkenes
• Alkenes can be prepared from alcohols and alkyl halides by
elimination reactions.
1- Dehydration of Alcohols
Examples:
13
Which alkene Predominates?
How do you which one is major product?
Saytzeff’s Rule:
Hydrogen is preferably removed from the carbon with least
no. of hydrogen since the alkene formed is more highly branched and
is energetically more stable.
Examples:
14
2- Dehydrohaloganation of Alkyl halides
In similar way to that in the dehydration of alcohol, Saytzeff’s rule
again applies; that is, the alkene with the most alkyl substitution on
the double-bonded carbons predominates.
Examples:
15
Saytzeff orientation:
Ease of formation of alkenes:
R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2
Stability of alkenes:
R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2
16
3- Dehaloganation of vicinal dihalides
alcohol
Example:
17
Reaction of alkenes
1- Addition reactions on the carbon-carbon double bond.
2- Substitution reactions on the saturated alkyl chain.
Addition reactions
Substitution reactions
1- Halogenation at High temperature
18
Substitution reactions
Halogenation at High temperature
C
C
Addition reaction:
low temperature
Absence of light
X2
C
or
H
Substitution reaction:
High temperature
X2 or Ultraviolet light
19
Addition reactions
1- Addition of Hydrogen: Catalytic Hydrogenation
Example:
20
2- Addition of Halogens: Halogenation
Examples:
Br
Br
21
3- Addition of Hydrogen Halides: Hydrohalogenation
Examples:
22
Markovnikov’s Rule
With unsymmetrical alkene
In the addition of an acid to an alkene the hydrogen will go
to the vinyl carbon that already has the greater number of
hydrogens.
23
Anti Markovnikov’s Rule
In the addition of an acid to an alkene the hydrogen will
go to the vinyl carbon that already has the lowest number
of hydrogens.
24
4- Addition of Sulfuric acid
heat
(H-OSO3H)
Addition of Sulfuric acid to alkenes also follows Markovnikov’s rule,
as the example
H2O
heat
Propene
Isopropyl hydrogen sulfate
2-Propanol
25
5- Addition of Water : Hydration
Addition of HOH across the double bond is in accordance
with Markovnikov’s rule, as the example
26
6- Addition of HOX : Halohydrin Formation
The addition of the chloronium ion, Cl+ or bromonium ion, Br+ and
the hydroxide ion, OH-, follows the Markovnikov’s rule, as the example.
27
7- Oxidation of alkenes
1) Oxidation of alkenes with Permanganate (Baeyer test)
Examples:
syn hydroxylation
28
2) Oxidation of alkenes with peroxy acid
OH
C C
HCOOOH
C C
O
H3O
C C
OH
Epoxide
anti hydroxylation
Examples:
H3C HC CH CH3
HCOOOH
H
H
OH
C C
H3C
O CH3
H3O
H
H3C C C CH3
H
OH
2,3-butanediol
OH
HCOOOH
O
H3O
OH
trans-1,2-cyclohexanediol
29
8- Ozonolysis
Oxidation of alkenes by ozone O3
The ozonolysis reaction can be summarized by the following equation:
30
Examples:
31
9- Polymerization
A)
B)
32
Homework
1- Which of the following compounds can exist as cis-trans isomers?
Draw the structures of the geometric isomers.
a) CH2=CHCH2CH2CH3
b) CH3CH2CH=CHCH2CH3
2- Draw the structures of the following compounds.
a) trans-4-Octene
b) cis-1,2-Dichloropropene
3- Name the following compounds using the E,Z .
H
a)
H3C
Cl
C
Br
b)
C
Br
Cl
C
I
C
F
4- Write the structure of the major product expected on reaction of
a) 1-butene with H2SO4
b) 2-methylpropene with aqueous bromine
c) 1-methylcyclopentene with water
d) The ozonolysis of 2-methyl-2-pentene
33